1. Field of the Invention
The present invention relates to a wireless communication apparatus, a wireless network system, a communication method and a program for communication between communication apparatuses in a wireless LAN system, etc.
2. Description of Related Art
In recent years, the WLAN network defined by IEEE 802.11 standard has expanded its market and usage (see Japanese Patent Application Publication Nos. 2004-312691, 2005-57601 and 2004-27248). In the past, an infrastructure network was built around a central control terminal, which is referred to as an access point. In recent years, however, it is more popular to use an ad hoc network, in which no access point exist and wireless terminals communicate with each other on an equal basis.
The ad hoc network is often formed with battery-driven apparatuses that serve as wireless terminals or nodes, and thus low power consumption or high power efficiency is strongly desirable. The IEEE 802.11 standard defines operations in the low power consumption state in the ad hoc network as follows in “11.2.2 Power management in an IBSS.”
Wireless terminals are classified into PS mode wireless terminals operating in a power saving (Power Save: PS) mode or a low consumption operation mode and Active mode wireless terminals operating in an Active mode. Each Target Beacon Transmission Time (TBTT) interval is divided into an Announcement Traffic Indication Message (ATIM) window of traffic generation and a time period following the ATIM window.
In a case where a destination of a unicast transmission frame is a PS mode wireless terminal, the wireless terminal once buffers the frame, transmits an ATIM frame to the PS mode wireless terminal in an ATIM window, and notifies the PS mode wireless terminal that the wireless terminal is buffering the frame. In a case where the PS mode wireless terminal is included in destination terminals to which the transmission frame is transmitted by way of broadcasting and multicasting, the wireless terminal once buffers the frame, transmits an ATIM frame by way of broadcasting or multicasting through an ATIM window, and notifies the PS mode wireless terminal that the wireless terminal is buffering the frame.
In the ATIM window the PS mode wireless terminal is in the Awake state and ready to receive the frame. Upon receiving a unicast ATIM frame destined for itself in an ATIM window, the PS mode wireless terminal sends a response frame to the sender and maintains the Awake state until the next TBTT. Further, the Awake state is also maintained until the next TBTT in the case of receiving the ATIM frame of the broadcast and related multicasting. After the ATIM window is ended, the PS mode terminal, which does not receive the related ATIM, frame shifts to an Doze state until the next TBTT. The wireless terminal which transmits the ATIM frame by way of broadcasting and multicasting and the wireless terminal which transmits the unicast ATIM frame, which receives the returned response frame, respectively transmit the buffered frames to the intended addresses during a time period between the ending of the ATIM window and the next TBTT.
Further, during each TBTT, all the wireless terminals that connected to the ad hoc network select random delay values within a predetermined range. The wireless terminal that selects the minimum delay value transmits a Beacon after the selected delay. In a case where the PS mode wireless terminal transmits the Beacon, the terminal keeps the Awake state until the next TBTT, even if no ATIM frame is transmitted or received. The Active mode wireless terminal always holds the Awake state.
As mentioned above, according to the IEEE 802.11 standard, information on whether or not there is a transmission frame is notified during the ATIM window, which exists in the head of the TBTT. Only when there is the transmission frame, the Awake state is maintained. Otherwise, the Doze state is selected, thus achieving the low power consumption operation.